Mathematical model of raw hide curing with brine

The most common method of preserving raw hidesis brine curing with sodium chloride. However, thisprocess has three important disadvantages: first, thelength of time that it takes, which is a minimum
…

The most common method of preserving raw hidesis brine curing with sodium chloride. However, thisprocess has three important disadvantages: first, thelength of time that it takes, which is a minimum of18 hours; second, the insufficient degree of curingreached in some hides due to an overload andpossibly the low efficiency of the brine raceway; andfinally, the environmental impact associated withthe discharge of large quantities of electrolytes in thesoaking step. Our long term goal is to address allthree issues. Initially, we have carried out a studyof the salt uptake and its diffusion mechanism inorder to attempt a reduction in the curing time. Acontinuous reaction mathematical model of a closedone dimensional system that describes the diffusionof sodium chloride in the hide during the curingprocess was chosen in the search for the optimumbrine curing conditions such as the optimum brineconcentration and percent float. The effect of thesetwo parameters on the values of transport coefficientwas reported. Brine diffusion into the hide wastracked by measurement of the chloride concentrationof the residual brine solution. In addition, a piece ofhide was cured with a fluorescently labeled brinesolution and analyzed by means of epifluorescentmicroscopy for direct visualization of the sodiumlocation within the hide.

Transcript

1.
167 MATHEMATICAL MODEL OF RAW HIDE CURING WITH BRINE by EDUARD HERNANDEZ BALADA l2, WILLIAM N. MAluviERl*, KAREL KOLOMAZNIK3, PETER H. Coo 1 AND ROBERT L. DUDLEY U.S. Department ofAgriculture, Agricultural Research Service, Eastern Regional Research Center 600 EAST MERMAID LANE, WYNDMOOR, PA 19038 USA Department of Chemical Engineering, University of Barcelona MARTI I FRANQUS 1, 08028 BARCELONA, SPAIN 3 Tomas Bata University MOSTNI 5139, 760 01 ZLIN, CZECH REPUBLIC ABSTRACT impacto medioambiental asociado con la descarga de grandes cantidades de electrolitos en la operación de The most common method of preserving raw hides remojo. Nuestro objetivo a largo plazo es resolver los is brine curing with sodium chloride. However, this mencionados problemas. Inicialmente, estudiamos la process has three important disadvantages: first, the absorción de sal y su mecanismo de difusión a fin de length of time that it takes, which is a minimum of reducir el tiempo de curado. Un modelo matemático 18 hours; second, the insufficient degree of curing de reacción continua de un sistema unidimensional reached in some hides due to an overload and cerrado fue escogido Para describir la difusión de possibly the low efficiency of the brine raceway; and cloruro de sodio en la pie1 durante el proceso de finally, the environmental impact associated with curado a fin de buscar las óptimas condiciones de the discharge of large quantities of electrolytes in the curado, tales como la concentración optima de soaking step. Our long term goal is to address all salmuera y el porcentaje óptimo de baflo. El efecto three issues. Initially, we have carried out a study de estos dos parámetros en los valores del coeficiente of the salt uptake and its diffusion mechanism in de transporte 2 son presentados. La difusión de sal en order to attempt a reduction in the curing time. A la pie] fue monitorizada mediante la determinaciOn continuous reaction mathematical model of a closed de concentración de cloruros en la solución residual one dimensional system that describes the diffusion de salmuera. Además, una muestra de piel fue of sodium chloride in the hide during the curing curada con una solución de salmuera etiquetada process was chosen in the search for the optimum con una sustancia fluorescente y analizada mediante brine curing conditions such as the optimum brine microscopla epifluorescente Para la visualización concentration and percent float. The effect of these directa del sodio en la pie1. two parameters on the values of transport coefficient was reported. Brine diffusion into the hide was INTRODUCTION tracked by measurement of the chloride concentration of the residual brine solution. In addition, a piece of Raw hides and skins are - 60-70% water and - 25-30% hide was cured with a fluorescently labeled brine protein. In this form the hide is susceptible to bacterial solution and analyzed by means of epifluorescent activity within hours after being removed from the carcass. microscopy for direct visualization of the sodium The autolytic degradation of skins/hides is assumed to be due location within the hide. to a combined action of tissue enzymes and bacteria, the latter requiring moisture to be viable. Curing is the process that RESUMEN provides an environment in which bacteria cannot survive. Several curing agents have been reported in the literature, e.g., El método más comi.in para preservar pieles crudas potassium chloride , 2 silica gel,- boric acid, and herbal-based es el curado con salmuera. Sin embargo, este products .6 Common salt, in spite of its inherent impact on proceso presenta tres desventajas: en primer lugar, the environment and the large amount required, is the most el tiempo que requiere, un mIriimo de 18 horas; popular and inexpensive material used to preserve hides and en segundo lugar, el insuficiente nivel de curado skins. A suitable improved method would yield savings in salt, alcanzado en algunas pieles debido a la sobrecarga y shipping and effluent treatment costs as well as a diminished baja eficiencia del tanque de curado; y finalmente, el environmental impact. Mathematical modeling has been* Corresponding Author - Email address: william.marmer@ars.usda.gov Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. Manuscript received November 14, 2007, and accepted for publication November 19, 2007 JALCA, VOL. 103, 2008

2.
168 MODEL OF RAW HIDE CURING c(hr)=rc0(r) V c,(r) (O,rj=O k) L1 I/I j flesh side OA b (1 >1Figure 1: Mathematical model of the curing process of a raw hide. Figure 2: Dimensionless sodium chloride concentration field within thereported to be a powerful tool in the optimization of processes hide during the curing process.such as soaking of salted cattle hides. 7 8 Curing has beenmodeled in substrates such as cheese" () and meat", but has notbeen studied for the particular case of raw hides. (2)The aim of this work is to develop and verify a mathematical ax dxmodel that describes the diffusion of sodium chloride in the c(x,O) = 0 (2a)hide during the curing process. Upon its verification, themodel is applied in the search of the optimum values of process c0(0)=c01, (2b)variables such as brine concentration, float percentage andcuring time. Equation (3) is valid under an ideal mass transfer from the bulk solution to the surface of the solid phase. THEORYWe propose a continuous reaction model to describe the c(b,x) = £ c, (r) (3)diffusion of sodium chloride from the bath containing brinesolution to the surface of the solid phase (hide). The model The introduction of dimensionless parameters (equations 4aassumes that salt will further diffuse into the hides inner to 4e) has been demonstrated to be a useful tool in the modelvolume where it will form a non-stationary concentration development.field (Figure 1). It also assumes that diffusion takes place onlyinto the flesh side and that hide parameters such as thickness,surface and properties of both hair and flesh sides will remain C (4a) = LC0p constant throughout the whole process. CID Curing can be considered as a counter diffusion in which C, (4b)sodium chloride soaks into the skin as water simultaneously opwashes out. Equation (1) describes a non-stationary onedimensional concentration field inside the inner volume of the X = (4c)solid phase, defined by Ficks second law. Boundary condition(la) assumes that sodium chloride diffuses into the hide from D xthe flesh side only. Terms are defined at the end of the paper. F 0 =--- (4d) ac D.---(x,r) O<x<b r>O Na=J2- (4e) The dimensionless time, also called Fouriers number [F0], (la) assesses the proximity of the process to the equilibrium, i.e. Ox equilibrium is reached when F 0 --- . The dimensionlessEquation (2) corresponds to a mass balance of a closed system soaking number [Na] expresses the ratio between the volumes ofin which salt flow at the hide surface is equal to accumulation liquid and solid phases. The replacement of the dimensionlessspeed of sodium chloride in the bath. Equations (2a) and parameters into the previous model leads to a new dimensionless(2b) are the initial boundary conditions (t = 0). They assume model (Eq. (5a) to (50).a null initial content of sodium chloride in the fresh hideand a constant initial value of brine concentration in thebath respectively.JALCA. VOL. 103, 2008

3.
MODEL OF RAW HIDE CURING 169 F0g - Na a e N. C(F) -2Na2 N 20- N10-.--< +Na (9) Figure 3 shows the curves of the integral average concentration for various values of soaking number. C) Determination of Diffusion Coefficients The value of effective diffusion coefficient of sodium chloride in the hide can be evaluated from experimental data. Crank" suggested an equation for the diffusion coefficients study at short times: = 1+Na ,j - 02 04 08 08 1 12 14 18 18 I CO P c0(t)Figure 3: Dimensional sodium chloride concentration field within the c0 , -c 0 (cc) Na hide during the curing process.for various soaking numbers. (10) From the mass balance O<X<l F. >0 V0 = C12 V + E CO. V (11) (5b) We get Q1, F,) = C O (FO) COPNa CO = Na+ c (12) (50 ax aFO Transport parameter ? is defined as a ratio of the effective diffusion coefficient D to pore half length (a) square. (0,F)=O 0 ax (5d) C(X,O) = 0 (5e) A D -- 13) - D - a2 ( C(0) = 1 (50 when the factor for the tortuosity of the pores:the analytical solution of which can be obtained by means of (14)Laplaces transformation: cos(Xg,)eT-o g"c(x F Na +2Na ?. is an important value from an engineering point of view since e+Na esin(g ) E cos(g,,)- -g Nasin(g) it includes two phenomena not considered in the presented 91, model, which are the transport of water from the hide to the (6) bath and the interaction between sodium chloride and waterWhere g0 are the roots of the transcendent equation (7). counter flows during the curing. Nag tg(g) = - ( g0 > 0) (7) EXPERIMENTAL EThe three dimensional concentration field graphic Materialscorresponding to Eq. (6) is shown in Figure 2. Fresh cow hides were purchased from a local abattoir. They were soaked for 2 h (with surfactant) and then fleshed.Replacing Eq. (7) into Eq. (6) and rearranging terms (for X = 1), Approximately 6 x 10 in (15 x 25 cm) pieces were cut andwe obtain an equation that illustrates the variation of brine stored at -20°C. They were thawed at 4°C just before use.concentration with time. Food grade sodium chloride of purity minimum 99.82% was obtained from US Salt Corporation (Watkins Glen, NY). All -F0 -g other chemicals were reagent grade and used as received. °- Na +2Na c0( F Na Nag s+Na+ (8) Methods Thawed hide was cut into square pieces of approximately 4 xIn addition, integration of Eq. (6) leads to Eq. (9) which 4 in (10 x 10 cm) with an average weight of .- 100 g. Theycalculates the optimal time in order to reach a certain were transferred to a Dose drum (Model PFI 300-34, Dose Maschinenbau GmbH, Lichtenau, Germany), and tumbled atcontent of sodium chloride in the skin, C (integral averageconcentration). JALCA, VOL. 103, 2008

4.
170 MODEL OF RAW HIDE CURING and 5pM of the fluorescent dye and gently agitated. A 2-3 mm wide slice of the sample was excised manually with a stainless steel razor blade (cutting from flesh surface toward the grain) at regular intervals of incubation time, then mounted onto Petri dishes for imaging, using a Leica MZ FUJI stereomicroscope (Leica Microsystems, Bannockburn, IL, USA) equipped for epifluorescence and with a model DC200 color charge couple device camera system at 2.5x magnification. Samples were - FIesI irradiated with blue (470/40 nm) and UV (360/40 nm) light, 4h 5h 24h 48h and images of the fluorescence were acquired at 0.1 (blue)Figure 4: Epifluorescent microscopic images of a cross section of a hide at and 0.44 (UV) seconds of exposure time. Control samples, different stages of curing. The hide was cured with 30% (w/v) immersed in 500% v/v nano-pure water and 5pM dye, were labeled sodium chloride solution. examined under the same conditions of concentration and time to assess the penetration of the fluorescent dye in the absence of salt, and a blank sample was also examined to evaluate possible autofluorescence of the untreated raw hide. 0.70 0.60 RESULTS AND DISCUSSION 0.50 Results and Discussion Epifluorescence Microscopy 0.40 The diffusion of labeled sodium ions into the cross section 0.30 of hide samples was followed by means of epi-fluorescent 0.20 microscopy. In Figure 4, increased fluorescence indicating 0.10 diffusion of salt started on the flesh side and gradually moved 0.00 toward the hair side. The lack of fluorescence development at 10 12 the hair side validated the mathematical assumption described 4 4rniuf) by Eq. (la); this can be attributed to the existence of a thin protective barrier of sebaceous oil.i4 The series of imagesFigure 5: Determination of transport parameter X from experimental demonstrate the advance of fluorescence due to sodium data. The graph corresponds to cOp = 30% (wlv) and Na = 3. ions into the cross sections of hide as well as increases in fluorescence intensity throughout curing time. Signal saturation in the area near the flesh side, denoted by a very6 rpm with brine solution for varying time intervals after which bright fluorescence, was observed after 5 h of curing. Thethey were pulled out of the drum, hand-squeezed to wipe excess apparent retrograde movement of the labeled sodium betweenwater, sealed in plastic bags and placed in the refrigerator. A 2 h and 5 h may be due to the shrinking of the hide caused byfraction of the residual brine solution was also collected at dehydration. Surprisingly, sodium ions did not seem to reachdifferent time intervals. Two sets of experiments were carried the hair side even after 48 h of curing. This could be due toout: a constant 300% (v!v) float (volume of brine solution/ many factors. In order to determine if the penetrability ofvolume of hide) at different initial salt concentration levels the dye is a technical factor, a sample of hide incubated in an(20%, 25%, 30% and 35% (wlv), which correspond to 64, aqueous solution of dye for 24 hours was examined under the80, 96 and 100°SAL, respectively) and a constant 30% (wlv) microscope and then transferred to a beaker with concentratedinitial salt concentration (weight of NaCl/volume of solution) brine solution for 24 more hours. The dye did not fluoresce inat different float percentages (300%, 500%, 750% and 1000% the uncured sample but showed a high fluorescence after beingv/v). Density of hide was assumed to be - lg/cm3. cured for 24 hours (graphs not shown). However, fluorescence was absent or undetectable in the upper part of the corium,Analyses leading to the possibility that the dye may not penetrate intoChloride concentration determination the tightly-woven and dense structure of the corium, possiblyChloride concentration was determined by classical Mohr due to its size (MW = 586 Da). The use of scanning electrontitration. 3 Residual brine samples were diluted (1:100 v/v) microscopy with energy dispersive X-Ray spectroscopy (SEM-in nano pure water prior to titration. All samples were run EDS) and elemental mapping to measure the amount andin triplicate. location of salt in a brine-cured hide is an alternative method to fluorescence imaging and this approach is planned.Fluorescence ImagingCoroNa TM Green Sodium Indicator fluorescent dye (Invitrogen, Determination of Diffusion CoefficientsCarlsbad, CA) was used as a probe of sodium ions diffusing The diffusion of salt in the hide was evaluated by means of theinto raw hide from brine solution. A piece of raw hide of transport coefficient X, which can be calculated from the slopeapproximately 1 x 1 in (2.5 x 2.5 cm) was immersed in a of the straight line that results from plotting C 0(t) versus squarebeaker containing 500% v/v float of a 30% w/v NaCl solution root of time (Eq. 10). Taking into account early publishedJALCA, VOL. 103, 2008

5.
MODEL OF RAW HIDE CURING 171 TABLE I Transport Coefficient ? for Various Conditions of Initial Brine Concentration (c0 ) and Soaking Number (Na) Na=3 Cop =30%(wlv) (% w/v) k iO (s) R2 Na ? 10 (s) R2 20 4.2 0.835 5.3 0.949 25 3.8 0.921 9.0 0.887 30 5.3 0.949 7.5 4.1 0.905 35 10.7 0.776 10 8.6 0.876a2 < critical value for a = 0.05.results 12 and the accuracy of our measurements, the linear common practice in tanneries, which operate at Na 4 evendependence holds approximately as far as to the value of C0(t) though the generally accepted rule requires a Na ^- 5 in order= 0.6. Figure 5 depicts this correlation for the particular case for hides to receive a proper cure) 8 In addition, a large floatof c11 = 30% (w/v) and Na = 3. will help maintain an almost constant salt concentration. The outstandingly low value of? obtained for Na = 7.5 may be dueAs seen in Table 1, all X values, except from that of c0=35% to factors inherent to the hide, e.g., poor fleshing, which slows(wlv), are on the order of 10 s. These results are of the same down salt penetration, agglutination of the fibers and contentorder of magnitude as those reported in the mathematical of dry matter.model of soaking 15, which suggests that the diffusion of saltdoes not significantly differ between curing and soaking. A Determination of Optimum Brine Curing Conditionsnumerical value of ?. for c11 =35% (wlv) may not be reliable, An 85% salt saturation of the water remaining in the hide wasbecause the brine was initially supersaturated and the model established as a minimum standard in order to attain a properwas developed for homogeneous solutions solely. Note than degree of cure. 19 One can calculate the theoretical minimuma saturated brine solution holds 31.7g of salt in 100 ml of soaking number needed to attain this saturation percentage insolution, (c0 ) at 25°C.16 the equilibrium, that is, at infinite time, and without further additions of salt into the solution. From Eq. (8), If T — thenA comparison of the individual values of X is not simple, since F - ; thus the second summand can be neglected, giving:they may be affected by some of the following factors: 1. Thethickness of the hide, which may vary throughout the process c Na o Cand exerts a strong effect on the value of?, as seen in Eq. (13). Coll s+Na (15)2. The pore length, which varies among the hides and is hardlymeasurable. 3. The dry matter content of the hide, the variation Replacing C = E Co and rearranging for Na,of which may extensively modify ?.. In fact, ? may drop upto two orders of magnitude between a wet and a dry hide. 15,17 C Na =4. The temperature, which affects the diffusion rate and was C I C op -c (16)sometimes difficult to keep at 25°C during the process. 5. Theinfluence of the error in the measurement, i.e. the difficulty Using C = 0.85 • C c o ,,, a porosity of E = 0.5 and co, = 30 %to measure a small chloride concentration diminution with (wlv), and assuming that all pores are filled up with brinethe Mohr method despite the very low coefficient of variation solution, a soaking number of 4.4 is needed to reach 85%(CV) found for this technique (< 1%). saturation. Solving Eq. (16) for Co. = 20 and 25% (wlv), negative values of Na are obtained, indicating the unfeasibilityEven so, one can draw the conclusion on the effect that both c0 to attain 85% saturation. On the other hand, the minimumand Na exert on the values of?.. Increasing values of c 0p yielded soaking number would drop to 2.8 if the cure was startedlarger values of ?. as a consequence of an increasing gradientconcentration between the solid phase and the solution, which out with a saturated brine solution (31.7 % (wlv), oris in accordance with Ficks second law of diffusion. This fact 100 SAL). Notice that these values depend on the porosity ofcorroborates a general practice applied in curing raceways, the hide, which varies from one to another and within itself.21where solid salt is periodically added to the brine solution to Therefore, slightly different values of minimum Nas would bekeep it close to saturation (a 97 SAL). The float percentage obtained using another value of porosity.also exerts a remarkable effect on the values of transportparameter 7. Larger floats yielded faster diffusion of salt Working out the value of c0 from Eq. (16) and usinginto the hide, even though that effect became less significant Na = 3, we received a minimum initial brine concentrationfor Na > 5. That experimental observation corroborates the of 30.8% (wlv) in order to achieve the target saturation level. JALCA, VOL. 103, 2008

6.
172 MODEL OF RAW HIDE CURINGBy means of Eq. (9), the plot of which is depicted in Figure DEFINITION OF TERMS3, one is able to calculate the curing time needed to reach an85% salt saturation in the hide. As just mentioned above, c: concentration of sodium chloride in the hide moisture, at athis level cannot be achieved for c0 = 20, 25 and 30% (wlv) distance x from the boundary (t > 0) [mol m3]and Na = 3. However, a time of 4.2 h is obtained for a 35% c o : concentration of sodium chloride in the bath (t > 0) [mol(w/v) supersaturated brine and Na = 3. The 85% saturation is m] Con: concentration of saturated sodium chloride solution atalso achieved in 4.9, 7.7 and 3.4 h if the hide is cured with a 25°C [mol m]brine of c0 = 30% (w/v) and Na = 5, 7.5 and 10, respectively. cop: initial concentration of sodium chloride in the bath (t =These values are substantially lower than the 18 hours that 0) [mol m3]usually are required for a full cure in a normal float (Na-5). c. equilibrium concentration of sodium chloride in the bathThe calculations of those times contain the parameter ?., - [mol m3]and therefore are affected by the same factors mentioned in C : dimensionless concentration integral average [1]the previous section. In spite of this, it is interesting to note D: diffusion coefficient of sodium chloride in the hide [m 2 s1]the decrease of curing time with increasing float percentages, D: effective diffusion coefficient of sodium chloride in theexcept from the erratic value obtained for Na = 7.5. hide [m 2 s1] S: outer surface of the solid phase (skin) [m2] CONCLUSIONS a: pore half length of the skin [m] b: thickness of cured hide [m]Over 20 millions brine-cured hides were exported by the U.S. Na: soaking number [1]in 2006 (U.S. Leather Industry Statistics, 2007). Increasing V: volume of skin [m3]commodity prices for sodium chloride over the past few years V0: volume of brine solution [m3]together with issues associated with water pollution set the F0: Fourier number/dimensionless time [1]alarm off in the leather and meatpacking industries. The X: dimensionless distance [1]purpose of research reported in this article was to optimize the C, C o : dimensionless concentrations [1]brine curing of hides and skins under specific process conditions Greek symbolsby means of mathematical modeling. The diffusion of salt into T: time (s)the hide was characterized by the transport coefficient X, which C: porosity of solid state [1]was found to be in the order of 10 s. The usage of saturated X: transport coefficient [s]brine as well as large floats (>500%) yielded higher values of?., therefore higher diffusion rates. From the model it was REFERENCESalso possible to determine the minimum float and initial brineconcentration needed to attain an 85% salt saturation in the 1. Rao, B.R., and Henrickson, L.; Short-term preservation ofhide. This saturation level was not achieved employing brines cattlehide. JALCJI 78, 48-53, 1983.of initial concentration of 20 and 25% (wlv) independently of 2. Bailey, D.C., and Gosselin, J.A.; The preservation of animalthe float percentage used. For 30 and 35% brines, a minimum hides and skins with potassium chloride. JALCA 91, 317-float of - 440% and -280% was found, respectively. Using 333, 1996.a 30% (w/v) brine, the targeted 85% saturation is attained 3. Kanagaraj, J . , Chandra Babu, N.K., Sadulla, S., Suseelain shorter times as the % float increases, and one may expect Rajkumar, G., Visalakshi V., and Chandra Kumar, N.;the same trend for any other initial brine concentration. The Cleaner techniques for the preservation of raw goat skins. J.established 85% salt saturation in the hide obviously plays a Clean Prod. 9, 261-268, 2001.critical role in the search for optimum conditions of curing, 4. Munz, K.H.; Silicates for raw hide curing. JALCA 102, 16-and the need to attain this saturation level for a proper cure will 21, 2007.be discussed in our next contribution. 5. Kanagaraj, J . , John Sundar, V., Muralidharan, C., and Sadulla, S.; Alternatives to sodium chloride in prevention ACKNOWLEDGMENTS of skin protein degradation—a case study. I Clean Prod. 13,The authors would like to thank Eleanor Brown, Laurelie 825-831, 2005.Bumanlag, Gary Di Maio, Rafael Garcia, Michael Kurantz, 6. Preethi, V., Rathinasamy, V., Kannan, N., Babu, C., andJoseph Lee, John Phillips, Maryann Taylor and Michaela Sehgal, P.K.; Azardirachta Indica: a green material for curingUhliffová for their technical support and assistance. of hides and skins in leather processing. JALCA 101, 266- 273, 2006. 7. OBrien, D.J.; A mathematical model for unsteady state salt diffusion from brine-cured cattlehides. JALCA 78, 286-299, 1983.JALCA, VOL. 103, 2008